Tin plate and container for carbonated beverages



United States Patent 3,367,751 TIN PLATE AND CONTAINER FOR '(IARBONATED BEVERAGES James W. Halley, Dune Acres, Ind., Leslie M. Bernick,

Calumet City, 11]., and William H. McFarland, Hobart,

Ind, assignors to Inland Steel Company, Chicago, IlL,

a corporation of Delaware No Drawing. Filed Aug. 18, 1965, Ser. No. 480,752 8 Claims. (Cl. 29--183.5)

This invention relates generally to an improved tin plate product and more particularly to a tin coated steel sheet suitable for use in carbonated beverage containers which are subject to attack by corrosion.

Carbonated beverage containers, such as the metal cans commonly used for packaging cola beverages and carbonated fruit flavored beverages, are usually made from tin plate. It has been found that the most significant factor in determining the storage life of a tin plate carbonated beverage can is the corrosion resistance of the steel base of the tin plate.

Heretofore there has been no meaningful correlation between the corrosion resistance of a carbonated beverage container and any single characteristic or specific combination of characteristics of the steel base of the tin plate forming the container. However, in tin plate containers for general use, i.e. for uses other than for carbonated beverages, it has been found that low carbon steels having low residuals usually exhibit better resistance to corrosion than other steels. Consequently, on the assumption that the same factors are also important in tin plate for carbonated beverage containers, an effort has been made to maintain a relatively low level of non-ferrous elements, particularly arsenic, copper, and nickel, in steel to be used for making tin plate for carbonated beverage containers. A typical steel composition heretofore used in tin plate carbonated beverage containers is as follows (wt. percent): 0.05-0.08 carbon, 0.30-0.40 manganese, 0.020 0.030 sulfur, 0.06 copper, .015 phosphorus, 0.04 nickel, 0.02 arsenic, and .010 silicon.

Despite the use of steel having the foregoing composi- -tion, however, most tin plate carbonated beverage containers wherein the beverage contains a strong acid, such as phosphoric acid, and an organic dye, such as an azo dye, or a carbonated fruit flavored beverage, such as grape soda, will fail because of corrosion of the steel base in less than twelve months of normal storage.

In addition, it has been observed generally heretofore that when large or massive inclusions are present in the steel base of tin plate, there is an adverse efiFect on the can making or forming properties of the steel. Since inclusions in steel are usually present as metal oxides or sulfides, it has been common practice in the container industry to specify a relatively low sulfur content for the steel to be used in making tin plate containers.

We have now found, quite unexpectedly, that the usual concepts regarding steel analysis and the role of inclusions do not apply in the case of tin plate containers for carbonated beverages and that it is necessary to depart drastically from the prior practices in the tin plate con- 3,36 7,75 l Patented Feb. 6, 1968 tainer field in order to realize consistently good corrosion resistance in carbonated beverage containers.

It is, therefore, a primary object of the present invention to provide a novel and improved tin plate product having improved corrosion resistance.

A further object of the invention is to provide a novel corrosion resistant tin plate product in which the steel base has a low carbon content and a low residual content and also contains higher than normal amounts of elements which are known to form inclusions.

An additional object of the invention is to provide a novel corrosion resistant tin plate product in which the steel base has a low carbon content and a low residual content and also contains abnormally high amounts of sulfur and manganese.

Another object of the invention is to provide a novel corrosion resistant tin plate product comprising a steel base having a high inclusion count.

Still another object of the invention is to provide a novel tin plate product comprising a steel base which is highly resistant to attack by carbonated beverages.

It is also an object of the invention to provide an improved tin plate product having improved corrosion resistance for use in the manufacture of carbonated beverage containers.

Other objects of the present invention will be apparent from the detailed description and claims: to follow.

It has now been found that a steel having a carefully controlled chemical composition so as to form a large number of relatively fine inclusions (i.e. a high inclusion count) consistently results in the production of a tin plate product having high corrosion resistance to carbonated beverages, such as colas and carbonated fruit flavored soft drinks. More particularly, a low carbon steel containing (on a wt. percent basis) a maximum of .09% carbon, a maximum of .04% copper, from about 0.40% to about 0.70% manganese, and from about .035 to about .075 sulfur consistently has a high inclusion count and exhibits high resistance to corrosion when incorporated in a tin plate carbonated beverage container.

Steel strips of the foregoing composition, when cold rolled and annealed in the usual manner and electrolytically tin coated in accordance with standard electroplating procedures, have been found to contain a higher than average number of relatively fine inclusions without any significant number of large diameter or massive inclusions. Thus, it has been found unexpectedly advantageous to provide in tin plate for carbonated beverage containers a steel base having a large number of relatively fine inclusions without, however, also forming very large or massive inclusions. And, while the present invention is not dependent on any particular theory of operation, it is postulated that with a large number of relatively fine inclusions in the steel base of the tin plate a small galvanic cell is formed around each of the small inclusions throughout the steel base and that these small galvanic cells in the steel base cause less damage to the steel base when the tin plate is in prolonged contact with a carbonated beverage than where there are a number of very large inclusions or where there are only an average or less than average number of inclusions in the steel base.

To provide tin plate which consistently displays high corrosion resistance when fabricated into carbonated beverage containers or the like, in accordance with the present invention, it is necessary to control carefully the chemical composition of the steel so as to insure the desired higher than average number of relatively fine inclusions without also having a high incidence of large diameter inclusions. It has been found, for example, that a low carbon steel having a maximum carbon content of .09 wt. percent and a maximum copper content of .04 wt. percent with the usual small amounts of other residual elements will have the desired high inclusion count when the manganese and sulfur content of the steel are both increased above the usual levels. Preferably, these higher than usual levels range between about .40% and about 0.70% by weight for manganese and between about 035% and about .075 by weight for sulfur. These unusually high levels of both manganese and sulfur in combination with a low carbon steel having low residuals appear consistently to induce the formation of a large number of relatively fine inclusions, such as iron sulfide and manganese sulfide, as opposed to forming a few very large inclusions.

The inclusion count, or number of inclusions per square inch, in the steel base of a tin plate carbonated beverage container is determined from a sample comprising a carefully polished unetched transverse section. Those skilled in the art will understand that during cold rolling of the steel or tin plate strip the inclusions in the steel base are caused to elongate in the direction of rolling. Accordingly, to determine the number of inclusions, it is important that the count be made on the sample edge which is transverse to the direction of rolling. The inclusions in six square inches of the polished transverse edge of the sample are counted at a magnification of 250x, each inclusion count value preferably representing the average of three random determinations in dififerent areas of the sample edge. When the foregoing procedure is followed, an inclusion count in excess of about twenty (20) per square inch at 250x is indicative of high corrosion resistance to attack by a carbonated beverage.

As used herein, the term inclusion count is intended to refer to relatively fine or small diameter inclusions rather than large diameter or massive inclusions, as previously described. In general, the tin plate used in carbonated beverage containers will have a thickness ranging from about .0050" to about .0075, usually from about .0061" to about .0066. A relatively large or massive inclusion is regarded as one whose size or diameter, when observed in accordance with the aforementioned inclusion count technique, is equal to or greater than about of the thickness of the sample.

The following specific examples are given to illustrate the present invention without, however, limiting the invention to the precise proportions and conditions indicated.

Example 1 A steel was prepared as a control (Test 1) having a sulfur content of .027 wt. percent, a manganese content of .36 wt. percent, and a carbon content of .07 wt. percent. Mold additions were made to two ingots of the above steel (Tests 2 and 3) in order to increase the manganese and sulfur content thereof in accordance with the present invention.

manner utilized in the normal production of tin plate and corresponding to the following illustrated treatment:

Pickled Hot Rolled Band .0800" Cold Reduce To .0087

Wash l Box Anneal 4 hrs. at 1250 F.

Cold Reduce To .0061" Electrolytic Tinning mined by a second test utilizing a standard corrosive solution containing =l20 p.p.m. dye (PD and C blue No. l) and 0.124 wt. percent phosphoric acid. In the test 475 ml. of this solution is placed in a 500 ml. Erlenmeyer flask fitted with a three-hole rubber stopper containing two gas inlet tubes and one gas outlet tube. One inlet tube is one inch from the bottom of the flask while the other inlet tube and the outlet tube are flush with the bottom of the stopper. The solution is first degassed through the longer inlet tube with a rapid flow of CO for approximately minutes. After degassing, the gas fiow is switched to the short inlet tube to maintain a C0 atmosphere. A sample (0.8 in. by 5.0 cm.) which has been electrolytically detinned and dealloyed in HCl is weighed, bent into the shape of a ring, and dropped into the flask. After immersion for a period of 96 hours at 80 F. the sample is removed and the weight loss determined. The corrosion rate in microamps. per square centimeter is obtained by multiplying the weight loss in milligrams by 1.25.

Experience has shown that a corrosion rate by the cola test of not more than about 10 microarnps. per square centimeter is indicative of tin plate having excellent corrosion resistance to carbonated beverages. Similarly, a dye test value of not more than about microarnps. per square centimeter, preferably not more than about 15 microarnps. per square centimeter, is also indicative of tin plate having excellent corrosion resistance.

The inclusion count values for the test samples were determined in the manner hereinabove described.

The test results are summarized in the following Thereafter the steels were processed in the conventional 65 Table I:

TABLE I Analysis, wt. percent Corrosion Rate Inclusion Test No. Count 0 Mn 8 Cu Cola Test Dye Test. (Nd/in! (amps/em!) (amps/0111. at OX) From the foregoing data it will be seen that the steels of Tests 2 and 3 having high sulfur and manganese contents and a high inclusion count, in accordance with the present invention, had superior corrosion resistance as compared to the control sample of Test 1.

Example 2 Another test series was run to evaluate the properties of a tin plate (Test 6) made in accordance with the invention but having slightly lower sulfur and manganese contents than in Example 1. Two control samples (Tests 4 and 5 were utilized in this instance.

The test results are summarized in the following root beer, and grape, orange, strawberry, lemon, lime, and grapefruit soda or similar fruit flavored beverages.

We claim:

1. A corrosion resistant tin plate comprising a low carbon steel base containing from about 0.40 wt. percent to about 0.70 wt. percent manganese and from about .035 wt. percent to about .075 Wt. percent sulfur.

2. A tin plate as in claim 1, wherein said steel base has an inclusion count in excess of about. inclusions per square inch when examined at 250x.

3. A corrosion resistant tin plate for use in carbonated beverage containers comprising a low carbon steel base with a high inclusion count containing about .09 wt. per- Table II: cent max. carbon, about .04 wt. percent max. copper,

TABLE II Analysis, wt. percent Corrosion Rate Inclusion Test No. Count Mn S Cu Cola Test Dye Test (No/in. (amps/cm (ampsJcmfi) at 250X) As will be evident, the steel of Test 6 had excellent corrosion resistance as compared to the control samples of Tests 4 and 5.

Example 3 A low carbon steel was prepared in accordance with the present invention having .09 wt. percent carbon, .58 wt. percent manganese, .039 wt. percent sulfur, and .04 wt. percent copper and was processed into tin plate in the conventional manner as described in Example 1. Thereafter, samples of the test steel were subjected to the cola corrosion rate test and inclusion counts were made in the usual manner. The cola corrosion rate was found to be 10 mi croamps. per square centimeter and the inclusion count was 21.4 per square inch at 250x.

It will be evident from the data of Examples 1, 2 and 3 that a tin plate having a steel base which exhibits an inclusion count above 20 also exhibits superior cola corrosion resistance and has a substantially lower dye test corrosion value than those steels having an inclusion count below 20. Each of the steel bases exhibiting the superior corrosion resistance to the carbonated beverage media also has a manganese concentration above about 0.40 wt. percent and not exceeding about 0.70 wt. percent and a sulfur concentration above about .035 wt. percent and not exceeding about .075 wt. percent, while having a maximum carbon concentration of about .09 wt. percent and a maximum copper concentration of about .04 wt. percent.

In the preceding description and in the claims to follow the term carbonated beverage is used to designate any soft drink beverage containing phosphoric acid or citric acid, with or without an organic dye, and which contains carbon dioxide gas under pressure, including the colas,

from about 0.40 wt. percent to about 0.70 wt. percent manganese, and from about .035 wt. percent to about .075 wt. percent sulfur.

4. A tin plate as in claim 3, wherein said steel base has an inclusion count in excess of about 20 inclusions per square inch when examined at 250x.

5. A corrosion resistant container comprising an electrolytically tin coated low carbon steel base containing from about 0.40 wt. percent to about 0.70 wt. percent manganese and from about .035 wt. percent to about .075 wt. percent sulfur.

6. A corrosion resistant container as in claim 5, wherein said steel base has an inclusion count in excess of about 20 inclusions per square inch when examined at 250x.

7. A corrosion resistant container for carbonated beverages comprising an electrolytically tin coated low carbon steel base containing about .09 wt. percent max. carbon, about .04 wt. percent max. copper, from about 0.40 wt. per-cent to about 0.70 wt. percent manganese, and from about .035 wt. percent to about .075 wt. percent sulfur.

8. A corrosion resistant container as in claim 7, wherein said steel base has an inclusion count in excess of about 20 inclusions per square inch when examined at 250x.

References Cited UNITED STATES PATENTS 2,116,107 5/4938 Erb 29---'l96.4 2,393,528 '1/1946 Goss et a1. 29'--196.4 3,193,417 7/1965 Kopchak -123 3,334,030 8/1967 Notman 29-4964 DAVID L. RECK, Primary Examiner. RICHARD O. DEAN, Examiner. 

1. A CORROSION RESISTANT TIN PLATE COMPRISING A LOW CARBON SET BASE CONTAINING FROM ABOUT 0.40 WT. PERCENT TO ABOUT 0.70 WT. PERCENT MANGANESE AND FROM ABOUT .035 WT. PERCENT TO ABOUT .075 WT. PERCENT SULFUR. 2.A TIN PLATE AS A CLAIM 1, WHEREIN SAID STEEL BASE HAS AN INCLUSION COUNT IN EXCESS OF ABOUT 20 INCLUSIONS PER SQUARE INCH WHEN EXAMINED AT 25X. 